Semiconductor cleaning, chemical treatment and drying technology has been well developed over the last 30 or so years. However, the devices and technology used to perform these processes are extremely expensive. Moreover, with the advent of more advanced lithography and other techniques and more stringent performance requirements of the ultimate design on the semiconductor wafer, the above processing techniques available presently will soon be unable to meet the needed processing requirements.
As above, with the increasing complexity of semiconductor devices, semiconductor wafers are increasingly vulnerable to multiple contamination sources. The sensitivity is due to the submicron feature sizes as well as the decreasing thickness of the deposited layers on the wafer surface. The minimum feature size being designed at present in dense integrated circuits is about 0.11 microns. This will soon shrink to less than a tenth of a micron. As the feature sizes and films become smaller, the allowable contaminant particle size also must be controlled to smaller and smaller dimensions. In general, the contaminant particle size should be about 10 times smaller than the minimum feature size, therefore requiring control of contaminant particulate matter to better than one-one hundredth of a micron (i.e. better than 10 nm).
Such physical dimensions make the eventual product very vulnerable to latent particulate contamination in the environment, both in the air (from the workers and equipment) and the materials used to process the semiconductor. For example, most of the substances used in the cleaning and chemical treatment processes, such as fluorides, solvents, acids, heavy metals, oxidizers, etc., are toxic or otherwise hazardous to both maintain and eliminate. Similarly, the high purity deionized water (DI water) typically used in existing processes is expensive to purchase and dispose of, as well as requiring specialized storage and distribution systems. Chemical treatment and cleaning operations are also sources of contamination. Such contamination results from both surface reactants as well as physical contamination, the latter of which may result from particulate that is delivered to the semiconductor wafer from the chemical treating and cleaning materials themselves no matter the purity of the product or may be transported from the components of the storage and delivery systems.
Furthermore, because semiconductor wafers are manufactured in batch processes and then stored until later processing, rather than being fabricated in a continuous process, they are even more susceptible to this contaminant particulate matter from the environment being introduced to the surface. Moreover, as the semiconductor wafers are dried at the end of each batch process so that they can be safely transported and stored safely, use of isopropyl alcohol, a solvent generally employed during drying of the semiconductor wafer, has become problematic due to its volatile emissions as well as other reasons.
Substances other than DI water and isopropyl alcohol may also be conduits for introducing contaminants to the semiconductor wafer. The contaminants on semiconductor wafer surfaces exist as films, discrete particles or groups of particles and adsorbed gases. These surface films and particles can be molecular compounds, ionic materials or atomic species. Contaminants in the form of molecular compounds are mostly condensed organic vapors from lubricants, greases, photo resists, solvent residues, organic components from deionized water or plastic storage containers, and metal oxides or hydroxides. Contaminants in the form of ionic materials may be cations and anions, mostly from inorganic compounds that may be physically adsorbed or chemically bonded, such as ions of sodium, fluorine and chlorine. Contaminants in the form of atomic or elemental species may be metals, such as copper, aluminum or gold, which may be chemically bonded to the semiconductor surface, or silicon particles or metal debris from equipment used in the processes.
Conventional cleaning technologies used to remove these various contaminants include brush scrubbing or megasonic processing. Although such technologies remove an acceptable amount of contaminants from the semiconductor wafers during processing at present, such methods are ultimately hard on the increasingly delicate structures. The mechanical energy associated with brush scrubbing and megasonic energy damage devices on the semiconductor wafer and introduce device dependent results. Direct contact between the relatively hard surface, like a brush, and the semiconductor wafer can transmit far greater force than necessary to remove the contaminants. Cleaning processes using only megasonic energy produce bubbles and waves that are less damaging to the substrate but may be effective to clean a limited size range of the contaminant particles. Another well known problem of megasonic cleaning is that of cavitation, where bubbles in the megasonic fluid collapse on the surface of the semiconductor wafer and thereby impart energy to the surface of the semiconductor wafer. This energy may destroy delicate/fine structures on the surface or may destroy the surface itself when the bubbles repeatedly collapse at the same location on the surface. In addition to these problems, such conventional techniques may not be able to remove enough contaminants in the future.
Another technology that may be used to clean semiconductor wafers involves using foam rather than a hard surface (brush scrubbing) or megasonic waves to contact the semiconductor surface. As shown in FIG. 1, a semiconductor wafer 100, enclosed within an apparatus 102, has a foam 106 containing bubbles 108 and liquid introduced to the surface of the semiconductor wafer 100 through a port 104. As the foam 106 decays and drains from the surface of the semiconductor wafer 100, the mass of bubbles 106 scrubs particles from the surface of the semiconductor wafer 100. Such a foam process may be improved to provide cleaning to the extent necessary for current or future semiconductor processing.